The present invention relates to low hysteresis exchange biased spin-valve and spin dependent tunneling magnetic field sensors.
Many kinds of electronic systems make use of magnetic material based devices. Digital memories are used extensively in digital systems of many kinds including computers and computer systems components, and digital signal processing systems. Such memories can be advantageously based on the storage of digital bits as alternative states of magnetization in magnetic materials in each memory cell, particularly in cells using thin-film magnetic materials, resulting in memories which use less electrical power and do not lose information upon removals of such electrical power.
Magnetometers and other magnetic sensing devices are also used extensively in many kinds of systems including magnetic disk memories and magnetic tape storage systems of various kinds. Such devices provide output signals representing the magnetic fields sensed thereby in a variety of situations.
Such memory cells and sensors can often be advantageously fabricated using ferromagnetic thin-film materials, and are often based on magnetoresistive sensing of magnetic states, or magnetic conditions, therein. Such devices may be provided on a surface of a monolithic integrated circuit to provide convenient electrical interconnections between the device and the operating circuitry therefor.
In the recent past, reducing the thicknesses of the ferromagnetic thin-films and the intermediate layers in extended “sandwich” structures in which the two major surfaces of the intermediate layer each have thereon an anisotropic ferromagnetic thin-film layer, including those having additional alternating ones of such films and layers, i.e. superlattices, have been shown to lead to a “giant magnetoresistive effect” being present. This effect yields a magnetoresistive response which can be in the range of an order of magnitude or more greater than that due to the well-known anisotropic magnetoresistive response.
In the ordinary anisotropic magnetoresistive response, varying differences between the direction of the magnetization vector in the ferromagnetic film and the direction of the sensing current passed through the film lead to varying differences in the effective electrical resistance in the direction of the current. The maximum electrical resistance occurs when the magnetization vector in the film and the current direction are parallel to one another, while the minimum resistance occurs when they are perpendicular to one another. The total electrical resistance in such a magnetoresistive ferromagnetic film can be shown to be given by a constant value, representing the minimum resistance, plus an additional value depending on the angle between the current direction in the film and the magnetization vector therein. This additional resistance follows a square of the cosine of that angle.
As a result, operating external magnetic fields can be used to vary the angle of the magnetization vector in such a film portion with respect to the easy axis of that film portion. Such an easy axis comes about because of an anisotropy in the film typically resulting from depositing that film in the presence of a fabrication external magnetic field oriented in the plane of the film along the direction desired for the easy axis in the resulting film. During subsequent operation of the device with the resulting film, such operating external magnetic fields can vary the angle to such an extent as to cause_switching of the film magnetization vector between two stable states which occur as magnetizations oriented in opposite directions along that easy axis. The state of the magnetization vector in such a film portion can be measured, or sensed, by the change in resistance encountered by current directed through this film portion. This arrangement has provided the basis for a ferromagnetic, magnetoresistive anisotropic thin-film to serve as part of a memory cell and to serve as part of a magnetic field sensor.
In contrast to this arrangement, the resistance in the plane of a ferromagnetic thin-film is isotropic with respect to the giant magnetoresistive effect rather than depending on the direction of a sensing current therethrough as for the anisotropic magnetoresistive effect. The giant magnetoresistive effect has a magnetization dependent component of resistance that varies as the cosine of the angle between magnetizations in the two ferromagnetic thin-films on either side of an intermediate layer. In the giant magnetoresistive effect, the electrical resistance through the “sandwich” or superlattice is lower if the_magnetizations in the two separated ferromagnetic thin-films are parallel than it is if these magnetizations are antiparallel, i.e. directed in opposing directions. Further, the also present anisotropic magnetoresistive effect in very thin-films is considerably reduced from the bulk values therefor in thicker films due to surface scattering, whereas very thin-films are a fundamental requirement to obtain a significant giant magnetoresistive effect.
In addition, as indicated, the giant magnetoresistive effect can be increased by adding further alternate intermediate and ferromagnetic thin-film layers to extend the “sandwich” or superlattice structure. The giant magnetoresistive effect is sometimes called the “spin valve effect” in view of the explanation that a larger fraction of conduction electrons are allowed to move more freely from one ferromagnetic thin-film layer to another if the magnetizations in these layers are parallel than if they are antiparallel with the result that the magnetization states of the layers act as sort of a valve.
These magnetizations results often come about because of magnetic exchange coupling between the ferromagnetic thin-films separated by the intermediate layers, these intermediate layers typically formed from a nonferromagnetic transition metal as an electrical conductor. The effect of the exchange coupling between the ferromagnetic thin-film layers is determined to a substantial degree by the thickness of such an intermediate layer therebetween. The effect of the coupling between the separated ferromagnetic thin-film layers has been found to oscillate as a function of this separation thickness between these layers in being ferromagnetic coupling (such that the magnetizations of the separated layers are parallel to one another) and antiferromagnetic coupling (such that the magnetizations of the separated layers are opposed to one another, or antiparallel to one another). Thus, for some separation thicknesses, the layer coupling can be of zero value between extremes of such oscillations.
Exhibiting the giant magnetoresistive effect in a superlattice structure, or in an abbreviated superlattice structure formed by a three layer “sandwich” structure, requires that there be arrangements in connection therewith that permit the establishment alternatively of both parallel and antiparallel orientations of the magnetizations in the alternate ferromagnetic thin-film layers therein. One such arrangement is to have the separated ferromagnetic thin-films in the multilayer structure be antiferromagnetically coupled but to a sufficiently small degree so that the coupling field can be overcome by an external magnetic field.
Another arrangement is to form the ferromagnetic thin-film layers with alternating high and low coercivity materials so that the magnetization of the low coercivity material layers can be reversed without reversing the magnetizations of the others. A further alternative arrangement is to provide “soft” ferromagnetic thin-films and exchange couple every other one of them with an adjacent magnetically hard layer (forming a ferromagnetic thin-film double layer) so that the ferromagnetic double layer will be relatively unaffected by externally applied magnetic fields even though the magnetizations of the other ferromagnetic thin-film layers will be subject to being controlled by such an external field.
One further alternative arrangement, related to the first, is to provide such a multilayer structure that is, however, etched into strips such that demagnetizing effects and currents in such a strip can be used to orient the magnetizations antiparallel, and so that externally applied magnetic fields can orient the magnetizations parallel. Thus, parallel and antiparallel magnetizations can be established in the ferromagnetic thin-films of the structure as desired in a particular use. Such a structure must be fabricated so that any ferromagnetic or antiferromagnetic coupling between separated ferromagnetic films is not too strong so as to prevent such establishments of film magnetizations using practical interconnection arrangements.
A magnetic field sensor suited for fabrication with dimensions of a few microns or less to tens of microns or more can be fabricated that provides a suitable response to the presence of very small external magnetic fields and low power dissipation by substituting an electrical insulator for a conductor in the nonmagnetic intermediate layer. This sensor can be fabricated using ferromagnetic thin-film materials of similar or different kinds in each of the outer magnetic films provided in a “sandwich” structure on either side of an intermediate nonmagnetic layer which ferromagnetic films may be composite films, but this insulating intermediate nonmagnetic layer permits electrical current to effectively pass therethrough based primarily on a quantum electrodynamic effect “tunneling” current.
This “tunneling” current has a magnitude dependence on the angle between the magnetization vectors in each of the ferromagnetic layers on either side of the intermediate layer due to the transmission barrier provided by this intermediate layer depending on the degree of matching of the spin polarizations of the electrons tunneling therethrough with the spin polarizations of the conduction electrons in the ferromagnetic layers, the latter being set by the layer magnetization directions to provide a “magnetic valve effect”. Such an effect results in an effective resistance, or conductance, characterizing this intermediate layer with respect to the “tunneling” current therethrough. The maximum fractional change in effective resistance is a function of the magnetic polarization of the conduction electrons given by(ΔR/R)≃2P1P2/(1+P1P2)where P1 and P2 are the conduction electron spin polarizations of the two ferromagnetic layers. These polarizations appear dependent on the ratio of spin up to spin down electrons in the 3D shell of the transition elements used in the_ferromagnetic thin-films, i.e. the spin polarization P of the conduction electrons. The fraction f of 3D electrons which are spin up have typical values of 0.75 for iron, 0.64 for cobalt and 0.56 for nickel. Conduction electrons in metals are normally S shell electrons which theoretically would be equally divided between spin up and spin down electrons. However, because of band splitting the conduction electrons in the magnetic layers are assumed to have a fraction of spin up electrons like that of the electrons in the 3D shell. The spin polarization is then determined from P=2f−1.
In addition, shape anisotropy is often used in such a sensor to provide different coercivities in the two ferromagnetic layers, and by forming_one of the ferromagnetic layers to be thicker than the other. Such devices may be provided on a surface of a monolithic integrated circuit to thereby allow providing convenient electrical connections between each such sensor device and the operating circuitry therefor.
A “sandwich” structure for such a sensor, based on having an intermediate thin layer of a nonmagnetic, dielectric separating material with two major surfaces on each of which a anisotropic ferromagnetic thin-film is positioned, exhibits the “magnetic valve effect” if the materials for the ferromagnetic thin-films and the intermediate layers are properly selected and have sufficiently small thicknesses. The resulting “magnetic valve effect” can yield a response which can be several times in magnitude greater than that due to the “giant magnetoresistive effect” in a similar sized sensor structure.
The current-voltage characteristics of such “sandwich” structure sensors will exhibit a relatively linear change in the quantum electrodynamic effect “tunneling” current therethrough from one ferromagnetic layer through the barrier to the other with respect to the voltage provided across the sensor, i.e. across the barrier layer between these ferromagnetic layers, for relatively lower value voltages, but the current magnitude increases more than linearly for higher values of voltage across the sensor. As the voltage across the sensor increases, the fractional change in the “tunneling” current through the sensor, for the ferromagnetic layers having magnetizations changing from parallel to one another to antiparallel, decreases to being only half as great with several hundred millivolts across the sensor as occurs in the situation with a hundred or less millivolts across the sensor so that this fractional change with sensor voltage will range from a few percent to 20% or more. The fractional change in the resistance of the sensor for the ferromagnetic layers having magnetizations changing from parallel to one another to antiparallel increases to about one and one-half the room temperature values when the sensor is cooled to 77° K, but the “tunneling” current through the sensor increases by only about 10% to 20% indicating that the effective resistivity of the sensor is relatively insensitive to temperature (around 500 to 1000 ppm/° C).
The effective resistivity of such a sensor is set by the amount of “tunneling” current through the cell permitted by the barrier layer therein for the voltage across the sensor. The high sensitivity of the “tunneling” current to the thickness of the barrier layer leads to a wide range of sensor resistivities which have been observed to be from less than 60.0Ω-μm2 to 10,000MΩ-μm2. On the other hand, the barrier layer appears to permit relatively little magnetic coupling between the ferromagnetic layers thereacross with the coupling fields typically being only a few Oe. the ferromagnetic layers thereacross with the coupling fields typically being only a few Oe.
Magnetoresistive spin valve sensors have been used in a variety of applications. These include, for example, magnetic read heads in magnetic disk memories, land mines detection and current sensing in conductors. A typical spin-valve sensor based on the “giant magnetoresistive effect” (GMR), linear in its magnetoresistance versus applied external magnetic field characteristic, is fabricated, as indicated above, as a stack of magnetic and other materials layers forming a magnetoresistor that is quite long relative to its width with the length extending in a straight line or following a crenelated pattern as is shown in FIGS. 1A and 1B. One ferromagnetic layer therein is a reference layer having its direction of magnetization relatively fixed, or “pinned”, with another ferromagnetic layer being provided therein as a “free” layer having its magnetization direction more easily rotated by external magnetic fields which are the fields intended to be sensed. This last layer is separated from the reference layer by a nonmagnetic material layer, which is an electrical conductor for a GMR device, along which magnetoresistance is controlled by the angle between the relative magnetization directions in the two ferromagnetic material layers. The pinned axis of the magnetization direction the pinned layer is fixed in the transverse direction, or across the magnetoresistor width, by exchange coupling with an antiferromagnetic material layer (e.g. IrMn, CrPtMn), while the easy axis of magnetization of the “free” layer lies along the magnetoresistor length.
FIG. 1A shows top view of several typical magnetoresistive spin-valve magnetic field sensors, 5, interconnected in sequence to form a series circuit portion by interconnections, 6. FIG. 1B shows a cross section view of one of these sensors. The stack of various kinds of material layers forming these magnetoresistor structures providing such a magnetoresistive spin-valve magnetic field sensor 5 is fabricated on a monolithic integrated circuit chip substrate, 10, that may contain monolithic integrated circuits to provide amplification, electrical power management, switching controls, and other circuit based operations.
A dielectric film, 11, typically 2000 Å of silicon nitride, is first deposited over substrate 10 to provide a smooth surface for further material layer depositions and to electrically insulate the magnetoresistor from what is contained in substrate 10 below. This layer and the subsequent layers forming magnetoresistor structure 5 are deposited by sputtering using RF diode sputtering or DC magnetron sputtering techniques in a vacuum deposition chamber. A buffer layer, 20, of tantalum 30 Å thick is first deposited on layer 11 followed by depositing permalloy, or NiFeCo, to form after etching a ferromagnetic material “free” layer, 24, that is 50 Å thick. Layer 24 is deposited in the presence of a magnetic field having a magnitude of 20 Oe in an initial selected direction to orient an induced easy axis in the layer in that direction.
Then a 30 Å thick copper layer, 25, is deposited on the free layer to form a nonmagnetic, electrically conductive intermediate layer on which a 40 Å thick cobalt layer, 26, is deposited to form after etching a “pinned” layer also in the presence of a magnetic field with a magnitude of 20 Oe in the initial selected direction. Layer 26 has its magnetization direction “pinned” by depositing an antiferromagnetic material (e.g., IrMn, CrPtMn) “pinning” layer, 27, again in the presence of a 20 Oe magnitude magnetic field in the initial selected direction, the antiferromagnetic material being CrPtMn to a thickness of 350 Å. A 50 Å thick tantalum interconnect buffer layer, 28, is provided on pinning layer 27 to protect the magnetoresistive structure during subsequent fabrication steps, and to facilitate good electrical contact to aluminum interconnection structures 6.
Following the provision of these layers, an annealing step is undertaken. The substrate and the stack are heated in the presence of a magnetic field with a magnitude of 3000 Oe in the initial selected direction with this field maintained during a one hour heating at 250° C. in forming gas and during the subsequent cooling to strengthen the pinning of layer 26 by layer 27, and to reduce the dispersion of the angular orientations of the easy axes from the initial selected direction over the extents thereof.
Magnetoresistor structures 5 are formed from this stack of deposited layers through a patterning process along with any other magnetoresistors being formed on the substrate. An etching mask of silicon nitride is provided through using patterned photoresist as an initial etching mask for patterning the silicon nitride in a Reactive Ion Etcher (RIE), and then using the resulting silicon nitride “hard mask” as an etching mask in an ion mill. The ion mill removes all materials in the deposited electrically conductive layers uncovered by the “hard mask”, and so exposed to the etching, as these materials in layer 28, uncovered by the mask, and in each layer below those portions of layer 28 are etched away down to silicon nitride layer 11 on substrate 10 so that the lengths of magnetoresistor structures resulting are parallel to the initial selected direction. Much of the silicon nitride hard mask is removed in the ion mill as well.
A passivating silicon nitride layer, 13, is deposited with sputter deposition over the magnetoresistor structure 5 to a thickness of about 2000 Å. Photolithography is used to form an etching mask for using reactive ion etching to cut contact holes in passivation layer 13. Aluminum interconnection metal is deposited over the remaining portions of passivation layer 13 and into the contact holes to a thickness of about 1500 Å. This aluminum layer is patterned using a photoresist etching mask and reactive ion etching again. A final passivating layer, 15, is provided by sputter deposition to a thickness of 1.5 μm.
Again, an annealing of the resulting magnetoresistors is performed, first, in the presence of a magnetic field with a magnitude of 3000 Oe in the initial selected direction now along the lengths of the magnetoresistors with this field maintained during a one hour heating at 240° C. in forming gas and during the subsequent cooling to reduce the dispersion of the angular orientations of the easy axes from the lengths of the magnetic material layers over the extents thereof. A further annealing step follows in the presence of a magnetic field with a magnitude of 3000 Oe perpendicular to the initial selected direction, and so along the widths of the resulting magnetoresistors, with this field maintained during a two hour heating at 240° C. in forming gas, and then at 265° C. for one hour, to reorient the pinned direction of layer 26 to be along the width of the magnetoresistors.
The annealing is completed in a further step in the presence of a magnetic field with a magnitude of 3000 Oe parallel to the initial selected direction, and so along the lengths of the resulting magnetoresistors, with this field maintained during a two hour heating at 160° C. in forming gas and during the subsequent cooling to reduce the dispersion of the angular orientations of the easy axes in free layer 24 from the initial selected direction over the extent thereof but at a reduced temperature to avoid affecting the direction of pinning set in layers 26 and 27. These last two annealing steps result in a pinning direction orientation at some relative angle to the widths of the magnetosistors to thereby provide a component of the interlayer coupling along the lengths thereof to provide some bias to aid in minimizing the device hysteresis.
Plots of the high externally applied magnetic field range and the low externally applied magnetic field range response characteristics of a typical spin valve are shown in the graphs of FIGS. 2A and 2B, respectively. The device resistance versus externally applied magnetic field response characteristics of a magnetic tunnel junction are qualitatively similar. However, the magnitudes of the resistance values and the resistance change values may be quite different. FIG. 2B shows that at moderately high positive externally applied magnetic fields the device resistance is largest, corresponding to the antiparallel alignment of the magnetizations of free and fixed layers 24 and 26; and the device resistance is smallest for moderately high negative externally applied magnetic fields, corresponding to the parallel alignment of the magnetizations of free and fixed layers 24 and 26.
FIG. 3 shows a graph in which the resistance of the device of FIG. 1 as an approximate fraction of its maximum resistance versus the angle between the magnetizations of free and fixed ferromagnetic layers 24 and 26 on either side of intermediate layer 25. This relationship is obtained by applying an external magnetic field along the direction indicated by the angle that is larger than the magnetic saturation field of free layer 24 but less than the magnetic saturation field of fixed layer 26.
Similarly, a typical spin dependent tunneling sensor, also linear in its magnetoresistance versus applied external magnetic field characteristic, is again fabricated as a stack of magnetic and other materials layers forming a magnetoresistor that is quite long relative to its width with the length extending in a straight line or following a serpentine or crenelated pattern. Here, too, as can again be represented in FIGS. 1A and 1B, one ferromagnetic layer therein is reference layer 26 having its direction of magnetization relatively fixed, or “pinned”, with another ferromagnetic layer being provided therein as “free” layer 24 having its magnetization more easily rotated by external magnetic fields. This last layer is separated from the reference layer in this instance, however, by electrically insulative material barrier layer 25, typically aluminum oxide 15 Å thick, through which electron tunneling is controlled by the angle between the relative magnetization directions in the two ferromagnetic layers. Once again, the easy axis of the magnetization of the pinned layer is fixed in the transverse direction, or across the magnetoresistor width, by exchange coupling with antiferromagnetic material layer 27 (e.g. IrMn, CrPtMn), while the easy axis of magnetization of the “free” layer lies along the magnetoresistor length.
Patterned, micron-size exchange biased spin-valve sensors exhibit quite different giant magnetoresistance effect responses as compared to the stack of sheet films from which the sensors are formed by patterning due to the strong magnetostatic interaction between the ferromagnetic layers in the resulting patterned sensors. This is also true of spin dependent tunneling sensors. The effect of the fringing fields from the pinned ferromagnetic material layer in such sensors detrimentally affects the external applied magnetic fields induced magnetization reversals of the free layer which causes a degradation in the linearity of its magnetoresistance versus applied external magnetic field characteristic, or linearity, of these sensors, and also an unwanted bias point shift of the free layer on such characteristics which limits the dynamic response range of the sensors.
The major challenges in making linear spin-valve sensors are to reduce the hysteresis and optimize the bias point. The general techniques used are biasing the sensor with an external field generated either by permanent magnets or patterned coils with driving currents. Thus, to ensure linearity, a longitudinal bias magnetic field is usually applied to overcome the influence of the demagnetization fields from both the pinned and free layers to thereby induce quiet and single domain behavior for the free layer. The general rule to achieve an optimized bias point for linear spin-valve sensor with a single free layer is to make a balance of the fringe field from the pinned layer with ferromagnetic interlayer coupling between the pinned and the free layers. However, all such measures complicate the fabrication process and usually lead to the resulting sensor consuming an undue amount of electrical power.